1. Field of the Invention
The present invention relates to polymer-derived ceramics, precursors, and methods of using and making the same.
2. Description of Related Art
Polymer-derived ceramics, such as silicon carbide (SiC), siliconoxycarbide (SiOC), silicon carbonitride (SiCN), aluminum nitride (AlN), and hafnium carbide (HfC), can be synthesized by thermal decomposition of suitable polymeric precursors, and possess some remarkable properties, such as high oxidation resistance, high temperature piezoresistivity, high mechanical strength, and photoluminescence One advantage of polymer-derived ceramics is that their microstructure can be modified on a molecular scale through modification of the polymer precursor. Most silicon-based polymer-derived ceramics are amorphous ceramics, prepared by controlled heating of polysilazane- or polysiloxane-based liquid polymeric precursors. The final ceramic's chemical and physical properties are known to depend on the initial molecular arrangement of the polymeric precursor and processing conditions. Boron-doping of polymer-derived ceramics has also been shown to result in enhanced electrical conductivity and thermoelectric power. Early work on boron-modified polysilazane (precursor for SiCN ceramic) and polysiloxane (precursor for SiOC) ceramics was performed by European researchers, in which boron was introduced into silazanes typically by aminolysis of chloroborosilanes with methyl amine or by ammonolysis of single-source precursor (B[C2H4Si(CH3)Cl2]3). Hydroboration of new dopants (HBCl2.SMe2) further enhanced the boron-to-silicon molar content in the precursor to 1:2 with 56% polymer to ceramic yield. Alternatively, dehydrogenative coupling reactions of hydrosilanes have also been utilized to synthesize polysilazanes or polycarbosilazanes polymer precursors. As a further simplification, hydroboration of vinyl-substituted polysilazanes has also been performed after ammonolysis of chlorosilanes, to eliminate the by-product salt formed in the polymer. In almost all of these methods developed for Si(B)CN synthesis, preparation of the polymeric precursor requires multiple steps involving exclusive synthesis setup and machinery, and special handling of hazardous chemicals (e.g., borane dimethyl sulfide, chlorosilanes) and their by-products (e.g., ammonium chloride). They also contain impurities and precipitates that must be filtered out before use. Even the most recently-reported work on Si(B)CN polymeric precursor involved coammonoloysis of chlorosilanes with boron trichloride and results in a very low boron retention in the final ceramic. Moreover, current boron-doping techniques yield boron-modified silazanes that are in gel or semi-solid form upon incorporation of the boron, which limits their potential applications.
Polymer-derived ceramics have a number of different uses including the formation of protective coatings or ceramic matrices, as well as interfacing with various nanomaterials to form nanocomposites having high temperature stability, oxidation resistance, as well as enhanced electrical, electrochemical, and mechanical properties.